This invention relates generally to a method and apparatus for determining the amount of entrained air in freshly mixed concrete.
Deterioration of concrete structures due to freezing and thawing is one of the most significant problems in northern climates. Thus, major problems in concrete pavement such as scaling and spalling, pop-outs, D-line and pattern cracking, and the like are attributed to deteriorating effects of temperature extremes on concrete. Deterioration of concrete exposed to freezing and thawing is caused by hydraulic pressure generated by the expansion of freezing water in the capillary cavities of concrete. The magnitude of the hydraulic pressure depends on the distance between capillary pores and an escape boundary, such as an air void. In concrete, disruptive stresses will be developed, unless every capillary cavity in the paste is not farther than three or four thousandths of an inch from the nearest air void.
Ordinary concrete will contain a minimum of 1 percent of air voids. Experiments have indicated an expansion of 0.41 to 0.75 percent in concrete volume for a range of water to cement ratios at -4.degree. F. Therefore, the amount of empty space in nearly all concrete is large enough to accommodate the extra volume required by freezing of water in the capillaries. However, since the empty space is not sufficiently near to all capillaries, frost action would deteriorate the concrete. To overcome this problem, entrained air is provided.
Specifically, entrained air concrete contains a large number of very small air bubbles and is several times as resistant to frost action as non-air entrained concrete made of the same materials. Such entrained air concrete should be a dense, impermeable mixture that is well placed, protected, finished and cured if maximum durability is to be obtained. Preferably, the air voids are more effective when they are close together, and the cement paste in the concrete is normally protected against the effects of freezing and thawing if the spacing factor of the air void system is 0.08 inches or less, as determined with ASTM C 457.
The air content and size distribution of air voids produced in air entrained concrete are influenced by many factors, including but not limited to, the nature and concentration of the air entraining admixture, the nature and proportions of the constituents of the concrete mixture, the type and duration of mixing employed, the consistency, and the kind and degree of compaction applied in placing the concrete.
Therefore, it is very important to control the quality of air entrained concrete during mixing and placing. More particularly, it is very important to provide a rapid and reliable procedure for determining the air void characteristics, particularly the distribution of entrained air in freshly mixed concrete. No method, to date, is known for such a rapid, reliable and non-destructive method of in-place monitoring of entrained air in concrete.
Since the percentage of entrained air in concrete must be carefully controlled because the freezing and thawing durability is impaired if the concrete contains an insufficient amount of air and the strength is unnecessarily reduced if the percentage of air becomes excessive, some testing still must be performed for the same. Some common methods used to determine the air content in freshly mixed concrete are the volumetric method, gravimetric method and the pressure method.
In the volumetric method, a known volume of concrete is removed from the mix and mixed with water. The mixture is then agitated until the air separates from the slurry and a decrease in volume is then measured.
In the gravimetric method, the percentage of air is determined from the inverse relationship between the unit weight of concrete and the amount of entrained air.
In the pressure method, air which has been pumped to a predetermined pressure in a compartment of known volume is released into a sealed container full of concrete. The pressure-volume relationship or Boyle's law is then used to measure the amount of air.
However, none of these known tests can be used in situ. Rather, all of these methods must be performed in the laboratory. Further, such laboratory tests take anywhere from between one-half to one hour to perform. By that time, the concrete has already dried in place and is difficult to remove if the laboratory tests indicate that the air content is not satisfactory. Further, the in situ characteristics of the air entrained concrete may be quite different from the laboratory measured amount. Further, it is desirable to be able to measure the amount of entrained air at several locations in a pavement slab while it is being placed, in order to assure uniformity of the mix throughout the pavement. This cannot be performed with the laboratory tests.